It is known in the art relating to permanent magnet motors to control the torque of the motor by controlling the current supplied to the motor. Typically, an interior permanent magnet motor includes a stator having multiphase windings and a rotor having permanent magnets located internally which are surrounded by pole pieces. The magnetic field of the stator produced by the stator windings interacts with the magnetic field of the rotor, causing the rotor to rotate. The rotor magnetic field is produced by permanent magnets.
The rotation or torque of the motor is controlled by governing the magnitude and duration of the current applied to the stator windings. To generate the magnetic field, the stator current is switched on and off in each stator phase winding in a predetermined sequence synchronized with the rotor position. Usually, the stator current signal is a pulse width modulated signal applied by an inverter to the windings. A stator current command signal may be determined based upon a maximum torque per amp trajectory. After the desired trajectory is determined, current regulators maintain the current command signal at the stator windings. The current regulators may be a hysteretic type of current regulator or a proportional-integral PI current regulator. The motor is run most efficiently at the maximum torque per amp trajectory. Therefore, it is desirable to have a control system for a permanent magnet motor that minimizes the deviation from that maximum torque per amp trajectory.